The present disclosure relates to methods and systems for imaging gamma ray emission in a space, and more particularly, two and/or three dimensional gamma ray emission image reconstruction methods and applications thereof.
Radiation imaging techniques have been widely used in the field of diagnostic and therapeutic medical imaging. Among such techniques, nuclear medicine imaging involves imaging of a distribution of gamma ray sources, which are emitted from within a body. In nuclear medicine imaging, radiopharmaceuticals (i.e., gamma ray sources) are taken internally, for example, intravenously or orally, and external detectors are used to capture radiation emitted by the radiopharmaceuticals and to provide images. This method is distinguishable from diagnostic X-raying, wherein external radiation is passed through a body to form an image. Therefore, the nuclear medicine imaging is also referred to as an “emission imaging.”
Efforts have been made to utilize emission imaging techniques used in nuclear medicine to image radiation distributions in environment, for example, power plant monitoring, radiation waste management, cargo inspection, radiation contamination monitoring, environmental monitoring, etc.
One of the early research in this area was conducted by R. Redus, et. al. (see Reference 1 in References section below.) Redus et al. disclose a prototype imager, which combines a gamma ray imaging system with a conventional video camera, a personal computer-based data acquisition, and a display system. The gamma ray imager is based on a position sensitive photomultiplier tube (PSPMT) coupled to a segmented scintillator and collimator. The system superimposes a gamma ray image of a radioactivity distribution with a video image of the area, allowing a rapid and intuitive determination of a source location. This research led to a commercialization of “RadScan” series products by RMD instruments, LLC (MA, USA).
Other similar research includes “A portable gamma camera for radiation monitoring” by S. Gure et al. (see Reference 2 in References section below), which discloses a use of a multi-pinhole collimator; “Operations of the CARTOGAM portable gamma camera in a photon counting mode” (see Reference 4 in References section below); and “Development of coded-aperture imaging with a compact gamma camera” (see Reference 7 in Reference section below), which discloses a use of CCD based gamma detector and coded aperture collimator.
Other commercially available products for imaging radiological environment include “GammaCam™” from US Department of Energy (see Reference 3 in References section below) and “RadScan® 800” by BIL Solutions Ltd. (UK).
However, the proposed prior art methods and commercial products have not been widely accepted for imaging radiological environment because of some critical limitations.
Although the principle is similar, the conditions for imaging radiological environment are quite different from that of imaging a human in nuclear medicine. For example, environment imaging covers a much broader area and a distance between a target and a detector is substantially longer when compared to imaging a person. Typically, environment imaging is performed using a detector, which is placed few tens of meters to few meters away from a target. Further, gamma energy measurements in radiological environment imaging range up to few MeV (1,000,000 electron volt). On the other hand, a detector in nuclear medicine is configured to scan close to a contour of a human body and capture gamma rays ranging up to few hundreds of keV (1,000 electron volt.)
Due to such different imaging conditions, prior art environment imaging systems, which were based on nuclear medicine technologies for human imaging, posed some fundamental limitations, such as inferior sensitivity and spatial resolution. There are several factors that affect sensitivity of environment imaging systems:                Imaging a wide area from a long distance: sensitivity is inversely proportional to a distance square as shown in equation (1), where d is a distance between a detector and a radiation source/target. Thus, a long distance between a detector and a target is one of the major factors that cause a decrease in sensitivity of a detector system for radiological environment imaging.        
                    sensitivity        ≈                  1                      d            2                                              (        1        )                            Pinhole type collimation: a collimator is a necessary component for gamma emission imaging. It classifies directions of incoming gamma rays. However, in radiological environment imaging, where a relatively small gamma detector is used to cover a wide target area, pinhole or coded aperture type collimators are often used, which also contribute to decrease in the sensitivity of a detector system. Further, a heavier and thicker collimation required for higher energy gamma ray measurements in radiological environment also causes a sensitivity drop.        A scintillator is another essential component in a gamma imaging system, which converts gamma ray into visible photons. In order to sufficiently block incoming gamma ray, absorb their energy, and scintillate, a thickness of a scintillator should be configured according to an amount of incoming gamma ray energy. As such, a substantially thicker scintillator is required for a system for environment imaging, which measures substantially higher energy gamma ray compared to systems for nuclear medicine. However, the thickness of a scintillator is inversely related to an intrinsic spatial resolution in conventional gamma imaging systems. Thus, the thickness of scintillator may not be increased freely due to the spatial resolution trade off.        
Further, there are several factors that affect spatial resolution of environment imaging systems:                Imaging a long distance target: the spatial resolution of an imaging system is linearly proportional to a distance as shown in equation (2) where d is a distance between a detector and a radiation source. Thus, as a distance between a detector and a source increases, the resolution of an imaging system degrades.resolution≈d  (2)        Penetration: some portions of incoming gamma ray penetrate through a collimator shielding, especially around opening edges of a pinhole or coded aperture holes. Such penetration of gamma ray increases as incoming gamma ray energy increases. Thus, a decrease in spatial resolution due to gamma ray penetration is more significant in environment imaging systems, which involve higher energy gamma ray.        Thickness of scintillator: as discussed above, a thicker scintillator may improve sensitivity of an imaging system. However, thicker the scintillator, broader is the detector response function, which leads to resolution degradation.        
Since the sensitivity and resolution are conflicting parameters in a gamma ray imaging system, it is difficult to improve both parameters simultaneously. Consequently, no practical solution has been proposed from numerous previous attempts to develop methods and systems for the field of radiological environment imaging.